Most small molecule drugs bind enzymes or receptors in tight and well-defined pockets. On the other hand, protein-protein interactions are notoriously difficult to target using small molecules due to their large contact surfaces and the shallow grooves or flat interfaces involved. E3 ubiquitin ligases (of which hundreds are known in humans) confer substrate specificity for ubiquitination, and therefore, are more attractive therapeutic targets than general proteasome inhibitors due to their specificity for certain protein substrates. The development of ligands of E3 ligases has proven challenging, in part due to the fact that they must disrupt protein-protein interactions. However, recent developments have provided specific ligands which bind to these ligases. For example, since the discovery of nutlins, the first small molecule E3 ligase inhibitors, additional compounds have been reported that target E3 ligases but the field remains underdeveloped. For example, since the discovery of Nutlins, the first small molecule E3 ligase mouse double minute 2 homolog (MDM2) inhibitors, additional compounds have been reported that target MDM2 (i.e., human double minute 2 or HDM2) E3 ligases (J. Di, et al. Current Cancer Drug Targets (2011), 11(8), 987-994).
Tumor suppressor gene p53 plays an important role in cell growth arrest and apoptosis in response to DNA damage or stress (A. Vazquez, et al. Nat. Rev. Drug. Dis. (2008), 7, 979-982), and inactivation of p53 has been suggested as one of the major pathway for tumor cell survival (A. J. Levine, et al. Nature (2000), 408, 307-310). In cancer patients, about 50% were found with p53 mutation (M. Hollstein, et al. Science (1991), 233, 49-53), while patients with wild type p53 were often found p53 down regulation by MDM2 through the protein-protein interaction of p53 and MDM2 (P. Chene, et al. Nat. Rev. Cancer (2003), 3, 102-109). Under normal cell condition without oncogenic stress signal, MDM2 keeps p53 at low concentration. In response to DNA damage or cellular stress, p53 level increases, and that also causes increase in MDM2 due to the feedback loop from p53/MDM2 auto regulatory system. In other words, p53 regulates MDM2 at the transcription level, and MDM2 regulates p53 at its activity level (A. J. Levine, et al. Genes Dev. (1993) 7, 1126-1132).
Several mechanisms can explain p53 down regulation by MDM2. First, MDM2 binds to N-terminal domain of p53 and blocks expression of p53-responsive genes (J. Momand, et al. Cell (1992), 69, 1237-1245). Second, MDM2 shuttles p53 from nucleus to cytoplasm to facilitate proteolytic degradation (J. Roth, et al. EMBO J. (1998), 17, 554-564). Lastly, MDM2 carries intrinsic E3 ligase activity of conjugating ubiquitin to p53 for degradation through ubiquitin-dependent 26s proteasome system (UPS) (Y. Haupt, et al. Nature (1997) 387, 296-299). As such, because MDM2 functions as E3 ligase, recruiting MDM2 to a disease causing protein and effectuating its ubiquitination and degradation is an approach of high interest for drug discovery.
One E3 ligase with exciting therapeutic potential is the von Hippel-Lindau (VHL) tumor suppressor, the substrate recognition subunit of the E3 ligase complex VCB, which also consists of elongins B and C, Cul2 and Rbx1. The primary substrate of VHL is Hypoxia Inducible Factor 1α (HIF-1α), a transcription factor that upregulates genes such as the pro-angiogenic growth factor VEGF and the red blood cell inducing cytokine erythropoietin in response to low oxygen levels. The first small molecule ligands of Von Hippel Lindau (VHL) to the substrate recognition subunit of the E3 ligase were generated, and crystal structures were obtained confirming that the compound mimics the binding mode of the transcription factor HIF-1α, the major substrate of VHL.
Cereblon is a protein that in humans is encoded by the CRBN gene. CRBN orthologs are highly conserved from plants to humans, which underscores its physiological importance. Cereblon forms an E3 ubiquitin ligase complex with damaged DNA binding protein 1 (DDB1), Cullin-4A (CUL4A), and regulator of cullins 1 (ROC1). This complex ubiquitinates a number of other proteins. Through a mechanism which has not been completely elucidated, cereblon ubquitination of target proteins results in increased levels of fibroblast growth factor 8 (FGF8) and fibroblast growth factor 10 (FGF10). FGF8 in turn regulates a number of developmental processes, such as limb and auditory vesicle formation. The net result is that this ubiquitin ligase complex is important for limb outgrowth in embryos. In the absence of cereblon, DDB1 forms a complex with DDB2 that functions as a DNA damage-binding protein.
Inhibitors of Apotosis Proteins (IAPs) are a protein family involved in suppressing apoptosis, i.e. cell death. The human IAP family includes 8 members, and numerous other organisms contain IAP homologs. IAPs contain an E3 ligase specific domain and baculoviral IAP repeat (BIR) domains that recognize substrates, and promote their ubiquitination. IAPs promote ubiquitination and can directly bind and inhibit caspases. Caspases are proteases (e.g. caspase-3, caspase-7 and caspace-9) that implement apoptosis. As such, through the binding of caspases, IAPs inhibit cell death. However, pro-apoptotic stimuli can result in the release of mitochondrial proteins DIABLO (also known as second mitrochondria-derived activator of caspases or SMAC) and HTRA2 (also known as Omi). Binding of DIABLO and HTRA2 appears to block IAP activity.
SMAC interacts with essentially all known IAPB including XIAP, c-IAP1, c-IAP2, NIL-IAP, Bruce, and survivin. The first four amino acids (AVPI) of mature SMAC bind to a portion of IAPs, which is believed to be essential for blocking the anti-apoptotic effects of IAPs.
Bifunctional compounds such as those that are described in U.S. Patent Application Publications 2015-0291562 and 2014-0356322 (incorporated herein by reference), function to recruit endogenous proteins to an E3 ubiquiuin ligase for degradation. In particular, the publications describe bifunctional or proteolysis targeting chimeric (PROTAC) compounds, which find utility as modulators of targeted ubiquitination of a variety of polypeptides and other proteins, which are then degraded and/or otherwise inhibited by the bifunctional compounds.
An ongoing need exists in the art for effective treatments for disease associated with overexpression or aggregation of Rapidly Accelerated Fibrosarcoma (RAF), or the overactivation of RAF (such as constitutively active RAF). For example, current BRaf inhibitors (such as, vemurafenib and dabrafenib) only target V600 mutant BRaf. Thus, a need exists for diseases or disorders (such as, melanoma, lung cancer, pancreatic cancer, and/or colorectal cancers) that have different BRaf mutations that are insensitive to currently marketed agents. Furthermore, resistance mutations can emerge in response to BRaf/MEK inhibitor therapy. For example, the p61 splice variant can emerge in melanoma patients treated with BRaf/MEK inhibitor therapy, which leaves these patients with no clinical options. Currently marketed agents also bind to and cause paradoxical activation of wild-type BRaf, which results in clinical complications. In addition, the family of hypoactive Class III BRaf mutants that signal through heterodimerization with CRaf, constitute 40% of BRaf mutations in non-small cell lung cancer (NSCLC), and also appear sporadically across other cancers, cannot be targeted with any currently approved or clinical-stage BRaf inhibitors.
Thus, non-specific effects and the inability to target and modulate RAF, remain an obstacle to the development of effective treatments. As such, small-molecule therapeutic agents that effectively targets RAF (e.g., effectively inhibiting and/or degrading mutant forms of BRaf, while sparing wild-type BRaf) and that leverage or potentiate VHL's, cereblon's, MDM2's, and IAPs' substrate specificity would be very useful.